1. Field of the Invention
The present invention relates to a gear train of an automatic transmission for a vehicle that realizes a plurality of forward speeds, and in particular, seven forward speeds and reverse.
2. Description of Related Art
A conventional shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A gear train of such an automatic transmission changes rotating speed and torque received from a torque converter of the automatic transmission and transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. For that reason, an automatic transmission that enables more shift speeds is under constant investigation.
In addition, with the same number of speeds, features of a gear train such as durability, efficiency in power transmission, and size depend in part on the layout of combined planetary gear sets. Therefore, designs for a combining structure of a gear train are also under constant investigation.
A manual transmission that has too many speeds causes inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are generally more important for automatic transmissions because an automatic transmission automatically controls shifting operations basically with reduced manual operation by a driver.
In addition to various developments regarding four and five speed gear trains, six-speed automatic transmissions have recently been developed.
The above information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.